Most applications of single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), and multi-walled carbon nanotubes (MWCNT) often require that they are available in the form of dispersions in a purified form in a suitable solvent system. These types of carbon nanotubes are generically described as carbon nanotubes (CNT) unless otherwise indicated.
As produced raw carbon nanotube soots generally include material impurities (extraneous impurities), such as transition metal catalysts, graphitic carbons, amorphous carbon nanoparticles, fullerenes, carbon onions, and polycyclic aromatic hydrocarbons along with the desired carbon nanotube products. The nature and degree of the electronic impurities in a given raw material can depend on the method of synthesis, such as, for example, laser, arc, High-Pressure Carbon Monoxide Conversion (HiPco), chemical vapor deposition (CVD), or combustion.
Known purification protocols generally involve steps of generic unit operations like pre-oxidation, acid reflux, mechanical mixing, ultrasonication, filtration, neutralization, and centrifugation. Selecting a suitable combination depends upon the method of production of the carbon nanotubes and the specific impurity targeted. As shown below. Table 1 provides an exemplary list of the dominant impurities in different nanotube samples and unit operations employed in their purification.
TABLE 1CatalystDominantmetalcarbonUnit OperationsSI NoTube TypeimpuritiesimpuritiesEmployedYearReference1LaserCo, NiGraphiticHNO3 reflux,1998Rinzler et al.,neutralization,Appl. Phys.centrifugation, crossA 67, 29-37flow filtration(1998)2LaserCo, NiGraphiticgas oxidation, HCl2001Chiang et al.,washingJ. Phys.Chem. B 105,8297 (2001)3ArcNi, YGraphiticmicrowave exposure,2002HarutyunyanHCl washinget al., J. Phys.Chem. B 106,8671 (2002)4CVDCo, Fe, NiAmorphousair oxidation, HF1999Colomer etsupportedwashingal., Syntheticon zeolitesMetals 103,2482 (1999)5HiPCOFeFullerenes,wet air oxidation, HCl2002Sivarajan etAmorphouswashing and fluorinatedal., J. Phys.extraction of fullerenicChem. B 107,impurities1361 (2003)6HiPCOFeFullerenes,H2SO4 + HNO32004Wiltshire etAmorphoussonicational., ChemicalPhysicsLetters 386,239 (2004)7HiPCOFeFullerenes,one pot HCl + H2O22007Wang et al.,AmorphouswashingJ. Phys,Chem. B 111,1249-1252(2007)
A. G. Rinzler et al., “Large-scale purification of single-walled carbon nanotubes: process, product, and characterization,” Appl. Phys. A 67, 29-37 (1998) describes a large-scale purification approach for purifying carbon nanotubes employing a sequence of steps including, for example, nitric acid reflux, neutralization, centrifugation, and cross-flow filtration as essential steps to purify single-walled carbon nanotubes.
Extraneous impurities, such as catalyst metal particles, fullerenic carbon, amorphous carbon, graphitic carbon, and carbon onions, are present to different degrees in as prepared raw carbon nanotube samples. Oxidative chemical treatments as part of the purification protocol and multiple acid treatments as part of the typical purification processes result in reasonably clean carbon nanotubes (<0.5 wt % impurities). However, since the intrinsic electrical conductivity arises from the delocalized π electrons of the SWCNT for a SWCNT of a given length and diameter, an aggressive chemical purification or side-wall derivatization during the purification process drains the π electrons of the individual SWCNT. Such a loss of conductive electrons leads to a drastic fall in the single tube electrical conductance as well as the elimination of the inter-band optical transitions arising from the van Hove singularities. Accordingly, for many applications, especially applications requiring a combination of optical and electrical properties retaining the electronic structure of the CNT substantially intact is an important aspect in the formation of SWCNT inks.
There are numerous approaches that form stable dispersions of carbon nanotubes in water with the use of anionic, cationic, or non-ionic surfactants. These surfactants form a monolayer coating on the surface of the CNT in the dispersed form either as individuals or as thin bundles. There are also widely reported approaches that use ionic or neutral polymer molecules for solubilizing carbon nanotubes in a water medium. Known examples are, among others, polystyrene sulfonate, polyvinyl pyrrolidinone, polyethylene oxide (PEO), polypropylene oxide (PPO), and tri-block copolymers of PEO-PPO-PEO. However, when thin films of CNT networks are formed on solid substrates from such dispersions, most of the surfactants or the polymers remain as part of the carbon nanotube films/network as a coating on the carbon nanotubes and remain there even after treatments at elevated temperatures. Presence of such surface impurities affect the electronic properties of the carbon nanotube network—e.g., reducing the electrical conductivity of the network.
Another approach for forming carbon nanotube dispersions or inks in organic solvents is to chemically derivatize them. For example, Haddon et al., U.S. Pat. No. 6,331,262, describes an approach that involves end functionalization employing carboxylation followed by acid-chloride formation followed by the formation of amide linkage by reacting with a long chain amine. However, the resulting solutions in organic solvents did not show the characteristic absorption features in the UV-Visible range, thereby suggesting that the delocalized π electrons have been drained completely or significantly. In addition, it should be noted that the electrical properties of the functionalized carbon nanotubes were not reported.
Huang et al., U.S. Patent Publication No. 2006/0124028 A1, describes carbon nanotube ink compositions in an aqueous medium designed for inkjet printing, which were obtained by a chemical reaction involving an azo compound and carboxylated single-walled carbon nanotubes. This approach focuses on the ink-jet printability of the dispersion or ink rather than the intrinsic properties of the CNT altered by the azo-functionalization.
Carbon nanotube inks prepared, despite these prior art approaches, especially for SWCNT suffer from one or more of the following limitations:
a) Loss of inter-band optical transitions indicating a significant modification of the electronic structure of the simile-walled carbon nanotubes or electronic defects; and/or
b) Surfactant or polymeric dispersal aid residues that are not removable from the solid film when such inks are used to form carbon nanotube networks or films.
There is a need in the art for approaches that provide the formation of stable carbon nanotube inks in water or organic solvent media in which (a) the SWCNT have not lost their inter-band optical transitions signifying an intact electronic structure and (b) the dispersal aids that are used to stabilize the SWCNT do not leave non-volatile residue in the solid products such as films formed from such inks. Accordingly, it is desirable to provide solvent-based and water-based carbon nanotube inks that overcome these and other deficiencies of the prior art.
For example, in some embodiments, a dispersal aid system that is non-ionic, molecular in nature, conserves the electronic structure of the SWCNT as evidenced by the inter-band optical transitions and that uses dispersal aids that can be completely removed from the carbon nanotube network or films that are formed using the CNT ink.